Mammalian cell membranes are important to the structural integrity and activity of many cells and tissues. Of particular interest is the study of trans-membrane gated ion channels which act to directly and indirectly control a variety of pharmacological, physiological, and cellular processes. Numerous gated ion channels have been identified and investigated to determine their roles in cell function.
Gated ion channels are involved in receiving, integrating, transducing, conducting, and transmitting signals in a cell, e.g., a neuronal or muscle cell. Gated ion channels can determine membrane excitability. Gated ion channels can also influence the resting potential of membranes, wave forms, and frequencies of action potentials, and thresholds of excitation. Gated ion channels are typically expressed in electrically excitable cells, e.g., neuronal cells, and are multimeric. Gated ion channels can also be found in nonexcitable cells (e.g., adipose cells or liver cells), where they can play a role in, for example, signal transduction.
Among the numerous gated ion channels identified to date are channels that are responsive to, for example, modulation of voltage, temperature, chemical environment, pH, ligand concentration and/or mechanical stimulation. Examples of specific modulators include: ATP, capsaicin, neurotransmitters (e.g., acetylcholine), ions, e.g., Na+, Ca+, K+, Cl−, H+, Zn+, Cd+, and/or peptides, e.g., FMRF. Examples of gated ion channels responsive to these stimuli are members of the DEG/ENaC, TRPV and P2X gene superfamilies.
Members of the DEG/ENaC gene superfamily show a high degree of functional heterogeneity with a wide tissue distribution that includes transporting epithelia as well as neuronal excitable tissues. DEG/ENaC proteins are membrane proteins which are characterized by two transmembrane spanning domains, intracellular N- and C-termini and a cysteine-rich extracellular loop. Depending on their function in the cell, DEG/ENaC channels are either constitutively active like epithelial sodium channels (ENaC) which are involved in sodium homeostasis, or activated by mechanical stimuli as postulated for C. elegans degnerins, or by ligands such as peptides as is the case for FaNaC from Helix aspersa which is a FMRF amide peptide-activated channel and is involved in neurotransmission, or by protons as in the case for the acid sensing ion channels (ASICs). The mammalian members of this gene family known to date are αENaC (also known as SCNN1A or scnn1A), βENaC (also known as SCNN1B or scnn1B), γENaC (also known as SCNN1G or scnn1G), δENaC (also known as ENaCd, SCNN1D, scnn1D and dNaCh), ASIC1a (also known as ASIC, ASIC1, BNaC2, hBNaC2, ASICalpha, ACCN2 and Accn2), ASIC1b (also known as ASICbeta), ASIC2a (also known as BNC1, MDEG1, BNaC1 and ACCN1), ASIC2b (also known as MDEG2, ASIC2b), ASIC3 (also known as hASIC3, DRASIC, TNaC1, SLNAC1, ACCN3 and Accn3), ASIC4 (also known as BNaC4, SPASIC, ACCN4 and Accn4), BLINaC (also known as hINaC, ACCN5 and Accn5), and hINaC. For a recent review on this gene superfamily see Kellenberger, S. and Schild, L. (2002) Physiol. Rev. 82:735, incorporated herein by reference.
There are seven presently known members of the P2X gene superfamily; P2X1 (also known as P2RX1), P2X2 (also known as P2RX2), P2X3 (also known as P2RX3), P2X4 (also known as P2RX4), P2X5 (also known as P2RX5), P2X6 (also known as P2RX6), and P2X7 (also known as P2RX7). P2X protein structure is similar to ASIC protein structure in that they contain two transmembrane spanning domains, intracellular N- and C-termini and a cysteine-rich extracellular loop. All P2X receptors open in response to the release of extracellular ATP and are permeable to small ions and some have significant calcium permeability. P2X receptors are abundantly distributed on neurons, glia, epithelial, endothelia, bone, muscle and hematopoietic tissues. For a recent review on this gene superfamily, see North, R. A. (2002) Physiol. Rev. 82:1013, incorporated herein by reference.
The receptor expressed in sensory neurons that reacts to the pungent ingredient in chili peppers to produce a burning pain is the capsaicin (TRPV or vanilloid) receptor, denoted TRPV1 (also known as VR1, TRPV1 alpha, TRPV1beta). The TRPV1 receptor forms a nonselective cation channel that is activated by capsaicin and resiniferatoxin (RTX) as well as noxious heat (>43° C.), with the evoked responses potentiated by protons, e.g., H+ ions. Acid pH is also capable of inducing a slowly inactivating current that resembles the native proton-sensitive current in dorsal root ganglia. Expression of TRPV1, although predominantly in primary sensory neurons, is also found in various brain nuclei and the spinal cord (Physiol. Genomics 4:165-174, 2001).
Two structurally related receptors, TRPV2 (also known as VRL1 and VRL) and TRPV4 (also known as VRL-2, Trp12, VROAC, OTRPC4), do not respond to capsaicin, acid or moderate heat but rather are activated by high temperatures (Caterina, M. J., et al. (1999) Nature. 398(6726):436-41). In addition, this family of receptors, e.g., the TRPV or vanilloid family, contains the ECAC-1 (also known as TRPV5 and CAT2, CaT2) and ECAC-2 (also known as TRPV6, CaT, ECaC, CAT1, is CATL, and OTRPC3) receptors which are calcium selective channels (Peng, J. B., et al. (2001) Genomics 76(1-3):99-109). For a recent review of TRPV (vanilloid) receptors, see Szallasi, A. and Blumberg, P. M. (1999) Pharmacol. Rev. 51:159, incorporated herein by reference.
The ability of the members of the gated ion channels to respond to various stimuli, for example, chemical (e.g., ions), thermal and mechanical stimuli, and their location throughout the body, e.g., small diameter primary sensory neurons in the dorsal root ganglia and trigeminal ganglia, as well data derived from in vitro and in vivo models has implicated these channels in numerous neurological diseases, disorders and conditions. For example, it has been shown that the rat ASIC2a channel is activated by the same mutations as those causing neuronal degeneration in C. elegans. In addition, these receptors are activated by increases in extracellular proton, e.g., H+, concentration. By infusing low pH solutions into skin or muscle as well as prolonged intradermal infusion of low pH solutions creates a change in extracellular pH that mimics the hyperalgesia of chronic pain. Furthermore, transgenic mice, e.g., ASIC2a, ASIC3, P2X3 transgenic mice, all have modified responses to noxious and non-noxious stimuli. Thus, the biophysical, anatomical and pharmacological properties of the gated ion channels are consistent with their involvement in nociception.
Research has shown that ASICs play a role in pain, neurological diseases and disorders, gastrointestinal diseases and disorders, genitourinary diseases and disorders, and inflammation. For example, it has been shown that ASICs play a role in pain sensation (Price, M. P. et al., Neuron. 2001; 32(6): 1071-83; Chen, C. C. et al., Neurobiology 2002; 99(13) 8992-8997), including visceral and somatic pain (Aziz, Q., Eur. J. Gastroenterol. Hepatol. 2001; 13(8):891-6); chest pain that accompanies cardiac ischemia (Sutherland, S. P. et al. (2001) Proc Natl Acad Sci USA 98:711-716), and chronic hyperalgesia (Sluka, K. A. et al., Pain. 2003; 106(3):229-39). ASICs in central neurons have been shown to possibly contribute to the neuronal cell death associated with brain ischemia and epilepsy (Chesler, M., Physiol. Rev. 2003; 83: 1183-1221; Lipton, P., Physiol. Rev. 1999; 79:1431-1568). ASICs have also been shown to contribute to the neural mechanisms of fear conditioning, synaptic plasticity, learning, and memory (Wemmie, J. et al., J. Neurosci. 2003; 23(13):5496-5502; Wemmie, J. et al., Neuron. 2002; 34(3):463-77). ASICs have been shown to be involved in inflammation-related persistent pain and inflamed intestine (Wu, L. J. et al., J. Biol. Chem. 2004; 279(42):43716-24; Yiangou, Y., et al., Eur. J. Gastroenterol. Hepatol. 2001; 13(8): 891-6), and gastrointestinal stasis (Holzer, Curr. Opin. Pharm. 2003; 3: 618-325). Recent studies done in humans indicate that ASICs are the primary sensors of acid-induced pain (Ugawa et al., J. Clin. Invest. 2002; 110: 1185-90; Jones et al., J. Neurosci. 2004; 24: 10974-9). Furthermore, ASICs are also thought to play a role in gametogenesis and early embryonic development in Drosophila (Darboux, I. et al., J. Biol. Chem. 1998; 273(16):9424-9), underlie mechanosensory function in the gut (Page, A. J. et al. Gastroenterology. 2004; 127(6):1739-47), and have been shown to be involved in endocrine glands (Grunder, S. et al., Neuroreport. 2000; 11(8): 1607-11). Therefore, compounds that modulate these gated ion channels would be useful in the treatment of such diseases and disorders.